Classifying crystallizers and systems



THOMAS C. CHENG CLASSIFYING CRYSTALLIZERS AND SYSTEMS Nov. 28, 1961 4 Sheets-Sheet 3 Filed Sept. 24, 1959 T5 TI T3 TEMPERATURE 7 6 m I M F ME MN 0 RZ m U 4 2 m m 11 A 1 1 I I I I II E L m E W C m m M I EE I5 MN m B B0 AZ 0 L ZOPFQQBZMOZOO FIG.8

LAB/LE c ZONE /METASTABLE ZONE UNDERSATURA TION ZONE INVENTOR.

THOMAS C. CHENG ATTORNEYS TEMPERA TURE ZOfn (kkZMQZOQ Nov. 28, 1961 THOMAS c. CHENG CLASSIF'YING CRYSTALLIZERS AND SYSTEMS 4 Sheets-Sheet 4 VACUUM Filed Sept. 24, 1959 FIG.9

CENTRIFUGE 0R FILTER WET PRODUCT N w m 0 MM E M W M WM 6 Z A0 I 15 E m F m L E w W w U T m M 0 w 6 o m w 5 0% 0 IO m E 8Y6 I43 I44 I45 I46 147 148 I49 I50 15/ 7 TEMPERA runs "F C United Smtes Patent 3,010,395 CLASSIFYING CRYSTALLIZERS AND SYSTEMS Thomas C. Cheng, Chicago, 111., assignor to General American Transportation Corporation, Chicago, 111., a corporation of New York Filed Sept. 24, 1959, Ser. No. 842,599 26 filaims. (Cl. 23-273) The present invention relates to crystallizing apparatus and systems and particularly to such apparatus and systems employing classification therein to produce uniform, controlled size crystals from crystallizable solutions.

It is a general object of the invention to provide an improved apparatus and system of the character noted wherein desupersaturization of crystallizable solution and crystal growth therein serves to form nuclei and fines and undersized crystals md crystal product in mother liquor, the apparatus comprising a shell having several stages of classification structure therein to remove the nuclei from the solution and to remove crystals smaller than the crystal product from the solution whereby to obtain maximum production of crystal product in mother liquor.

Another object of the invention is to provide an improved classifying crystallizer and system in which crystallizable solution in a main stream undergoes desupersaturization and crystal growth to form nuclei and fines and undersized crystals and crystal product in mother liquor, the crystallizer including structure for withdrawing from the main stream a first side stream containing substantially all of the nuclei shortly after formation thereof, for thereafter heating the first side stream to dissolve the nuclei, and for returning the first side stream to the main stream; and also including structure for withdrawing from the main stream a second side stream containing crystals smaller than the crystal product and for returning the crystals smaller than the crystal product to the main stream for additional crystal growth.

Still another object of the invention is to provide in a classifying crystallizer apparatus and system of the character noted, multiple stages of classification within the crystallizer shell whereby the solution undergoing crystallization encounters successive zones of classification, the structure defining the first classification zone creating a rate of solution flow sufiicient to remove nuclei and successive classification zones creating rates of solution flow suitable to remove crystals smaller than the crystal product, the rates of flow in successive crystallization zones being successively greater.

In connection with the foregoing object it is a further object of the invention to provide in crystallizing apparatus and systems of the character noted, successively increasing cross sections of the successive classification zones and successively increasing lengths of the successive classification zones.

Yet another object of the invention is to provide an improved classifying crystallizer apparatus and system which provides more economic production with a large throughput of crystallizable solution per unit length of time, and which gives trouble-free operation.

A further object of the invention is to provide in a classifying crystallizer apparatus and system of the type set forth an improved entry bafHe which admits the crystallizable solution in the form of an annular liquid curtain whereby to prevent improper formation of nuclei due to violent turbulence upon entry and to eliminate salt lumps formed by splashing.

A still further object of the invention is to provide an improved entry baffle of the character noted including structure to reduce vapor entra nment of particles and constantly to renew the surface of the crystallizable solution in the crystallizer by breaking the thin film of crystals thereon, the film being broken and renewed without splashing whereby to increase the free surface exposed for flashing.

Further features of the invention pertain to the partic ular arrangements of the elements of the apparatus and systems whereby the above-outlined and additional operating features thereof are attained.

The invention, both as'to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in combination with the accompanying drawings in which:

FIGURE 1 is a diagrammatic view in vertical cross section through one preferred embodiment of a classifying crystallizer made in accordance with and embodying the principles of the present invention;

FIG. 2 is a view in horizontal section through the crystallizer of FIG. 1 along the line 22 thereof;

FIG. 3 is a View in horizontal section along the line 33 of FIG. 1;

FIG. 4 is a view in horizontal section along the line 4lof FIG. 1;

FIG. 5 is a diagrammatic illustration of a crystallizer system embodying the present invention and incorporating the crystallizer of FIG. 1 therein;

FIG. 6 is a diagrammatic illustration of a crystallizing system embodying the present invention and incorporating therein a crystallizer comprising three classification stages and also embodying the present invention;

FIG. 7 is a concentration-temperature curve for a conventional vacuum crystallization process applied to a common salt;

FIG. 8 is a concentration-temperature curve for the system of FIG. 5 as applied to the common salt for which the concent ation-temperature curve is shown in FIG. 7;

FIG. 9 is a diagrammatic illustration of a system embodying the present invention and incorporating therein the crystallizer of FIG. 1, the system being particularly adapted for producing crystalline ammonium sulfate obtained from the reaction of ammonia gas and concentrated sulfuric acid; and

FIG. 10 is a temperature-concentration curve for the classifying crystallizer process carried out by the system of FIG. 9.

There is shown in FIGS. 1 through 4 of the drawings a classifying crystallizer 109 embodying the present invention and adapted to be incorporated in the crystallizing system of the present invention, the crystallizer M30 including a shell comprising a cylindrical body 101 on top of which is disposed a cylindrical vapor head 102 and below which is disposed a bottom 131. Mounted on one side of the vapor head 102 and communicating with the interior thereof is a solution inlet connection 193 which is adapted to be connected to a source of crystallizable solution. The solution entering through the connection 103 impinges upon an entry bafile 165 supported in the vapor head 102 by a plate 104. The plate 194 is formed substantially fiat and extends across the vapor head 102 above the conduit 103, the plate 104 being inclined at an angle of approximately 10 with respect to the horizontal, the higher side of the plate 104 being disposed adjacent to the conduit 103 and the lowermost portion of the plate 104 being disposed opposite and away from the conduit 103. The bafiie 105 is cylindrical in configuration and surrounds a complementarily shaped opening formed in the plate 164, the bafiie 105 being disposed eccentrically with respect to the vapor head 162 as may be best seen in FIG. 2, the bafiie being positioned farthest away from that portion of the Wall forming the vapor head 162 which carries the connection 1% and being disposed nearest to the '2 I.) portion of the wall disposed away from and opposite the conduit 103. Because of this arrangement the solution entering the connection 103 impinges upon the baffle 105 and is divided into two streams which flow around the baffle 105 and fall therefrom in the form of a continuous circular liquid curtain. The upper end of the vapor head 102 is closed by acurved wall 106 carrying a first outlet 107 adapted'to be connected to a vacuum or area of low pressure and a second outlet 103 adapted to be connected to monitoring controls and the like.

The lower end of the vapor head 102 joins a connecting wall portion 116 which in turn connects with the body 101 to form a fluid tight connection therebetween.

' Mounted in the bottom of the vapor head 102 is a first funnel 110 formed as a section of a truncated cone which converges downwardly to connect with a substantially cylindrical neck 111 extending downwardly therefrom. The walls of the funnel 110 are preferably inclined at an angle greater than the angle of repose of the material to be treated in the crystallizer 100 and for most materials can be formed at an angle of 60 with respect to the horizontal. Disposed about the funnel 110 and within the body 101 is a bafile 112 connected to the inner 'wall of the body 101 completely around the periphery thereof and extending upwardly and inwardly toward the funnel 110. ireferably the baffie 112 is formed as a section of a cone and the smaller diameter thereof is slightly greater than the external diameter of the funnel 110 at the closest point thereof to provide therebetween an annular passage 115. Mounted on the body 101 is a collector ring or manifold 113 which is positioned to collect solution which flows upwardly through the passage 115, the collector ring 113 being provided with an outlet connection 114.

Disposed Within the body 101 and below the funnel 110 is a second funnel 120 having the greater diameter thereof equal substantially to the internal diameter of the body 101 and secured thereto completely around the periphery thereof. The wall of the funnel 120 may be inclined with respect to the horizontal at the same angle as the w'all of the funnel 110. The funnel 120 extends downwardly and terminates in an opening having a smaller diameter than the opening at the bottom of the funnel 110 and is connected to a cylindrical neck 121 having a diameter less than the diameter of the neck 111 and having a length greater than the length of the neck 111. Disposed within the shell 101 and extending around the inner circumference thereof is a conical baffle 122 which extends upwardly and is provided with an opening therein spaced from the wall of the funnel 120 to provideian annular passage 125 therebetween, the area of the passage 125 being greater than the area of the passage 115 described above. A collector ring or manifold 123 similar to the collector ring 113 is provided to receive solution flowing through the passage 125 and the collector ring 123 is provided with an outlet connection 124.

The funnels 110 and 120 divide the crystallizer into upper, intermediate and lower chambers'providing two crystallization stages and two classification zones, as will be explained more fully hereafter.

The lower end of the body 101 joins a conical wall section 130 which connects with the cylindrical bottom 131. A peripheral tangential outlet 133 is provided for the bottom 131 and the lower end of the bottom 131 is closed by a dome shaped wall 132. The wall 132 is provided with an outlet connection 134 at the bottom thereof in general alignment with the necks 111 and 121 on the funnels 110 and 120, respectively.

Referring to FIG. of the drawings, there is shown a crystallizing system 500 embodying the present invention and incorporating therein the crystallizer 100 which has been described above. Initial feed solution containing dissolved material which is to be crystallized from the solution is fed (from a source not shown) through a pipe 501 containing a control valve 501a and a check valve 501b which permits flow through the pipe 501 only in the direction of the arrow. The outlet of the check valve 5011) is connected by a pipe 502 to the inlet side of a heater 503 (the outlet of the heater 503 being connected by a pipe 504 to the conection 103 on the vapor head 102 of the crystallizer 100. Solution entering through the connection 103 falls downwardly about the baffle 105 and solution is accumulated so that during normal operation of the system 500, the liquid level in the crystallizer is maintained a short distance below the lower edge of the bafile 105, the liquid level being indicated by the numeral 108. The outlet connection 114 is connected by a pipe 515 to the inlet of a pump 516 which pumps solution to a pipe 517 connecting with the pipe 502, the pipe 517 having a control valve 517a therein to control the flow of solution therethrough. The outlet connection 124 is connected by a pipe 525 to the inlet side of a pump 526 which discharges solution under pressure into a pipe 527 conected to the pipe 504, the pipe 527 having a control valve 527a. therein to control the flow of solution therethrough. The outlet connection 133 is connected by a pipe 535 to the inlet side of a pump 536 which is connected to pump solution into the pipe 537 connecting withthe pipe 504, the pipe 537 having a control valve 537a therein to control the flow of solution therethrough. The bottom outlet connection 134 is connected to a pipe 545 which in turn is connected to the input of a pump 546. The outlet of the pump 546 is connected by a pipe 547 to apparatus 550 for removing crystal product from mother liquor such as a centrifuge or filter, the pipe 547 having a control valve 547a therein. Wash water can be added to the apparatus 550 to wash the crystal product through a pipe 551 and the wet crystal product is removed at 552. The mother liquor is returned by a pipe 553 to the pipe 502, the pipe 553 having a control'valve 553a therein and a check valve 553k permitting flow of solution only in the direction of the arrow, i.e., from the pipe 553 to the pipe 502.

To begin operation of the crystallizing system 500, initial feed solution is fed via the pipes 501 and 502 under the control of the valve 501a to the heater 503 where the solution is heated to a temperature above the equilibrium temperature Within the crystallizer 100 so that a portion of the solution will evaporate upon entering the vapor head 102 whereby to render the remaining solution supersaturated. Accordingly, upon entry of the solution into the vapor head 102, at least a portion of the solution is vaporized and the vapors are withdrawn through the outlet connection 107 and the pipe 505. The hot solution entering the vapor head 102 strikes the baflie and flows therearound in both directions so that a continuous circular liquid curtain falls from the lower edge of the battle 105, and after accumulation of sutficient solution to raise the level to the operating point the curtain falls onto the upper surface 108 of the solution in the crystallizer 10-0. The solution in the liquid curtain is exposed to a pressure corresponding to an equilibrium temperature below'the temperature of the incoming solution and, accordingly, a portion of the solution will be evaporated or flashed and the vapor passes upwardly through the opening in the baffle 105 and the plate 104 to the outlet connection 107. The falling solution will quickly reach equilibrium temperature due to the character of the stream falling from the bafiie 105 and, accordingly, most of the vaporization or flashing takes place while the solution is falling rather than after the solution has reached the surface 108. As a result there will be little stirring and no splashing action at the surface 108 and the falling curtain of solution will strike the surface 108 substantially perpendicular thereto with no whirling or turbulent action. As a result there will be no improper formation of nuclei due to violent turbulence of solution flow from the baflle 105 and-there will be no salt lumps from splashing, thereby to lengthen the period between wash outs of the crystallizer 100 and to eliminate any problem of clogging in the various pumps and valves. The solution surface 108 is usually covered with a thin film of crystals which will be broken by the falling flow of incoming solution and thus provide a constant renewal of the surface 108 and thus produce additional free surface for flashing of vapor upwardly through the baffie 105.

The solution continues downwardly under the action of the various pumps from the surface 108 and the main solution stream flows downwardly through the funnel 110 and the funnel 120 to the bottom 131, this movement of the solution being accompanied by formation of nuclei and fines and undersized crystals and crystal product in mother liquor by desupersaturation of the solution and crystal growth. A first side stream comprising a magma of supersaturated solution with nuclei and crystals therein is drawn from the main stream through the outlet connection 114. More specifically, a portion of the solution flowing downwardly through the neck 111 is directed upwardly in a direction opposite to its prior direction of flow and through the passage 115 into the outlet connection 114. The funnel 110, the neck 111 and the baflie 112 in cooperation with the shell body 101 form a first classification stage, the volume between the bottom of the funnel 111 and the outlet connection 114 providing a first classification zone. The first side stream is removed through the connection 114 via the pipe 515 under the action of the pump 516 and via the pipe 517 under the control of the valve 517a to the pipe 502. The rate of flow of the first side stream is adjusted so that the rate of flow of solution through the first classification zone, i.e., upwardly from the bottom of the neck 111 pass the bafiie 112 and into the connection 114, is slightly greater than the terminal se tling velocity of the nuclei therein, whereby the nuclei will eventually be remove from the first classification zone by floating or being carried past the bafile 112 and pass outwardly with a portion of the solution through the connection 114 while the large particles sink through the first classification zone back to the main stream.

A second side stream comprising a magma of supersaturated solution and crystals smaller than crystal prodnot is withdrawn through the outlet connection 124 from a second classification stage formed by the funnel 120, the neck 121, the baflle 122 and the shell body 101. The portion of the crystallizer 100 disposed between the lower end of the neck 121 and the outlet connection 124 comprise a second classification zone within which crystals smaller than crystal product can be removed in the second side stream. More specifically, the pump 526 can be operated under the control of the valve 527a to draw the second side stream through the second classification zone at a velocity slightly greater than the terminal settling velocities of the fines and all crystals smaller than the predetermined and desired crystal product. As a result fines and crystals smaller than crystal product issuing from the bottom of the neck 121 will be carried upwardly through the second classification zone with the second side stream and pass outwardly through the connection 124 via the pipe 525, the pump 526 and the pipe 527 to the pipe 504 while product crystals fall back into the main stream and into the bottom 131,

When first starting up the crystallizing system 500, there will be no crystals of crystal product size present in the bottom 131 and therefore the remaining solution not withdrawn through the first and second side streams will be withdrawn in a third side stream comprising a magma of supersaturated solution and crystals smaller than crystal product through the connection 133 and fed via the pipes 535 and 537 under the action of the pump 536 back to the pipe 504, whereby all of the solution Will be recirculated. Eventually crystals of product size will be obtained in the bottom 131 and thereafter yet another stream can be withdrawn from the crystallizer through the bottom connection 134. This stream will comprise a magma of mother liquor having crystal product therein and will be conveyed to the centrifuge or filter 550 via the pipes 545 and 547 under the action of the pump 546. After separation of the crystal prodnot from the mother liquor, the mother liquor is returned through the pipe 553 and mixed with the initial feed solution in the pipe 501 and thence fed through the pipe 502 to the heater 503. The system can now be operated in a substantially steady state condition with crystal product being continually withdrawn with mother liquor through the bottom connection 134, the excess solution being evaporated and the vapors withdrawn through the outlet connection 107, and new feed solution being added as required to compensate for removal of solids as crystal product.

The various control valves and pumps are adjusted to obtain economic production of product crystals with a large throughput per unit length of time. Economic production is best achieved by providing a low degree of supersaturation of the crystallizable solution entering the first funnel 110 in the first stage, a subsequent slow release of supersaturation, the maintenance of an optimum magma density in the crystallizer 100 and optimum rate of circulation through the crystallizer 100, providing more seed surface as compared to prior apparatus and systems, and steady solution flow throughout. The rate of growth of crystals is a function of the surface area of existing crystals available for crystal growth and since nuclei proportionately present the greatest surface area for crystal growth, nuclei will preferentially grow to the size of fines thereby exhausting the available supersaturation and preventing undersized crystals from growing to crystal product size. Therefore it is essential to control and prevent the formation of nuclei in the solution within the crystallizer 100 or to remove the nuclei if formed so that the growth of nuclei will not exhaust the available supersaturation of the solution. The rate of formation of nuclei can be suppressed by re-cycling large volues of magma and mother liquor to obtain a small temperature change before and after flashing so as to minimize the supersaturation release per cycle of solution through the crystallizer 100.

The crystallizer system 500 is operated in this manner, i.e., a substantial portion of the solution is withdrawn through the outlet connections 114, 124 and 133 to be recirculated and the heater 503 is operated to provide only a small temperature change before and after flashing of the solution as it enters the vapor head 102 through the connection 103. Such a method of operation minimizes the formation of nuclei as does also the configuration and structure of the entry bafile 105 described above. Any nuclei that are formed are immediately withdrawn in the first side stream from the first classification zone through the pipe 515 to the pipe 502. The nuclei in the first side stream are dissolved in the heater 503 so that substantially no nuclei are present in the entering stream within the pipe 504. The heater 503 is of the tube type and is operated to provide a low temperature difference across the surfaces of the tubes thereof and to provide a low temperature change between the inlet pipe 502 and the outlet pipe 504 therefor. The elevation of the heater 503 is preferably chosen relative to the crystallizer 100 so that there will be no evaporation within the heater 503 so that there will be a minimum deposition of crystals within the heater 503 and thereby to preserve the high heat transfer coeflicient thereof.

In addition to minimizing the heat added to the solution mix in the pipe 502 by means of the heater 503, the solution mix in the pipe 504 is further cooled by the addition thereto of the second side stream derived from the second classification zone through the connection 124 via the pipes 525 and 527 and by the addition thereto of the bottom side stream from the connection 133 via .tals and mother liquor.

, 7 the pipes 535 and 537. As a result of the mixing of these various streams, the magma entering the vapor head 102 through the connection 103 has a temperature only slightly above the equilibrium temperature within the vapor head 102.

As'has been mentioned before there will be present within the crystallizer 1% four successive ranges of solid particle size, i.e., nuclei, fines, undersized crystals and crystal product, the nuclei being the smallest in size. The circulat on magma in the first side stream from the first flash consists of slightly supersaturated solution, nuclei and a few .fines. The circulation magma from the second stage flowing through the outlet connection 124 may consist of a few nuclei, fines, undersized crys- The side stream drawn through the outlet connection 133 may consist of fines, undersized crystals and mother liquor. The bottom stream Withdrawn through the outlet connection 134 consists of crystal product in mother liquor. The second side stream drawn through the outlet connection 124 and the bottom side stream drawn through the outlet connection 133, in addition to cooling the heated solution in the pipe 504, furnish the crystal seeds for subsequent crystal growth in the crystallizer 100. Thus the entry stream flowing into the vapor head 102 will contain crystals with a wide range of sizes, but substantially no nuclei, in a slight undersaturated solution. Flashing in the vapor head 102 will remove sufiicient solvent vapor to provide a slightly supersaturated solution which can supply the necessary material for growth of the seed crystals present. 7 r

The factors governing the classification of crystals in the first and second classification zones and the elutriation taking place in these zones are the density of the magma and crystals, the content of solids in the magma,

flow area in theannular openings 115 and 125. Since the fiow rate is usually in the steady region, the fiow velocity is'in the range of 1.5 feet per minute to 5 feet or minute for most solutions. The settling rate of nuclei, fines and crystals decreases as the particle size decreases, i.e., the terminal settling velocity is smaller for the smaller particle size and therefore a relatively low fiow velocity through the first classification zone and the annular opening 115 will be sufficient to remove nuclei While returning substantially all larger particles to the main stream and correspondingly higher flow velocities must be maintained in the second side stream through the annular opening 125 to remove the fines and undersized crystals from the second classification zone through the outlet conduit 124. To achieve efficient classification, relatively long settling times are required and therefore a longer classification zone must be utilized with higher fiow velocities to obtain the same degree of separation as compared to classification zones utilizing lower flow velocities. Accordingly, the vertical extend of the first classification zone, i.e., the

"space between the bottom of the neck 111 and the outlet connection 114, is shorter than the height of the second classification zone, i.e., the distance between the lower end of the neck 121 and the second outlet connection 124. As the crystals grow larger the tendency to collide with other crystals of difierent size is increased, therefore a larger cross section of flow area is desirable in classifying the larger crystals. Accordingly, the cross section of the second or lower classification zone is preferably formed greater than the cross section of the first or upper classification zone. I

Preferably the above desired construction and arrangement of the parts in the crystallizer 1% is accomplished by providing like angularity of the walls of the funnels liliand 120, and changing the size and shape of the necks 111 and 12-1. Preferably the diameter of the upper neck 11-1 is greater than the diameter of thel0werneck-121 and conversely the length of the upper neck 1.11 is prefthe crystal shape, the magma viscosity, the flow rate within the zone, the size of crystals to be removed, and the erably less than the length of the lower neck 121. The greater diameter of the upper neck 111 facilitates and accommodates the greater total flow of solution therethrough since all of the solution introduced into the vapor head 1 02 and not evaporated must flow therethrough.

, The neck 121 on the other hand can have a smaller diameter since the total volume to be passed therethrough is less than that through the neck 111 and more particularly the volume through the neck 121 is reduced by the amount drawn through the connection 114. The length of the necks 111 and 121 are determined by (1) the rate of ratio of solution flow through the neck to the solution flow of the side stream taken therefrom through the connection 114 or 124 (the ratio Q/F), (2) the ma ma solids content of the solution through the neck, and (3) the size of the crystals in the solution falling through the neck. in the lower classification zone, the Q/F ratio is the smallest, and both the crystals content and size are the highest, and therefore the length of the neck 121 must be greater than the neck 111. The longer neck 121 provides a longer classification zone in the second stage and also provides a classification zone having a greater cross sectional flow area whereby to reducethe probability of collision and to minimize the rubbing effect between the crystals in the heavy magma present in this zone. The classification efiiciency is rendered maximum by the above described proportion of the parts.

Since the fiow rate in each stage is controlled by means of the valves 517a, 527a, 537a and 547a, the magma solids content can be kept constant throughout the crystallizer 1%. The solids content in the magma will be the lowest at the upper or first stage and the highest in the bottom or second stage While the degree of supersaturation is highest at the top orfirst stage and lowest at the bottom or second stage. Because the rate of crystal growth is dependent upon the exposed surface area present, the rate of release of supersaturation Will be substantially the same in the first stage as in the second stage and substantially the same throughout the crystallizer 1%. This will insure that the crystallization process will roceed at a constant and steady rate.

The above described principles are equally applicable to crystallizers having more than two stages and in fact are applicable to crystallizers having any desired number of stages, the number of stages employed in any particular crystallizer depending upon the particular performance desired and the requirements placed upon the crystal product, such as purity, crystal size, the range of crystal size, etc. There is shown in PEG. 6 of the drawings a crystallizer system 6% incorporating therein a three stage crystallizer 631. The crystallize]? includes a shell 682 having three funnels 61%, see and 63% dividing the shell into an upper vapor head, a first crystallization stage disposed between the funnels iillland 62%, a second crystalization stage disposed between the tunnels 62%) and 630 and a third crystallization stage disposed between the funnel 630 and the bottom 633, each of the crystallization stages providing a corresponding classification zone. The vapor head is provided with an inlet connection 603 which is positioned to direct incoming undersaturated solution against a baths 695 identical in construction to the baifie described above. Aportion of the solution flashes and the vapor formed passes upwardly through the opening in the baffle 695 through an outlet connection 697 in an upper wall 666 closing the upper end of the vapor head. The connection 607 is in communication with a pipe 608 which in turn is connected to an area of reduced pressure.

' The three classification stages are provided with outlet connections 614, 624- and 634, respectively from top to bottom, and with weir rings 612, 622 and 632 in association therewith and spaced from the corresponding funnels 610, .620 and163ti'to provide an annular flow passage therebetween. Each of the funnels is provided with a cylindrical neck, the funnel e19 having a neck 611, the funnel 620 having a neck 621 and the funnel 630 having a neck 631. The diameter of the necks progressively decreases from top to bottom and the length of the necks progressively increases from top to bottom, whereby the vertical extent of the classification zones increases from top to bottom and the cross sectional fiow area in the classification zones increases from top to bottom. Similarly the flow rates in the classification zones are regulated so that the lowest flow rate is maintained in the upper or first classification zone and the highest fiow rate is maintained in the third or lowermost classification zone.

Initial feed solution is fed (from a source not shown) through a pipe 661 including a control valve 661a and a check valve 66112 to a pipe 662 communicating wi h a heater 663. The output of the heater is fed through a pipe 664 to the inlet connection 603 and the solution not vaporized in'the shell 602 falls downwardly through the several funnels in a main stream whereby nuclei, fines, undersize crystals and crystal product in mother liquor are formed by desupersaturation and crystal growth. A first side stream is taken from the main stream in the first classification zone through the outlet connection 614 and is conveyed by a pipe 615 to a pump 616 and from the pump 616 by a pipe 617 to the pipe 662, the pipe 617 having a control valve 617a therein. A second side stream is derived from the main stream drawn through the outlet connection 624, a pipe 625, a pump 626 and a pipe 627 to the pipe 664, the pipe 627 having a control valve 627a therein. A third side stream is drawn from the main stream through the third classification zone from the outlet connection 634 through a pipe 635, a pump 636, and a pipe 637 to the pipe 664, the pipe 637 having a control valve 637a therein. The remaining portion of the main stream fiows downwardly through a bottom outlet connection 633, a pipe 645, a pump 646 and a pipe 647 also connecting with the pipe 664 and having a control valve 647a therein. A side pipe 648 is also connected to the pipe 647 to withdraw mother liquor with crystal product therein from the pipe 647, the pipe 648 having a control valve 648a therein to regulate the flow of the mother liquor with crystal product therethrough. The pipe 648 is connected to suitable apparatus 650 for removing the crystal product from the mother liquor, the apparatus 650 being, for example, a centrifuge or a filter. Wash water, if required, is supplied through a pipe 651 to the apparatus 650 and the wet product is removed therefrom through a conduit 652. The mother liquor leaves the apparatus 650 through a pipe 653 which connects with the pipe 652, the pipe 653 having a control valve 653a and a check valve 653!) therein.

The crystallizer system 600 operates in substantially the same manner as the system 500 described above. However, better control of the product and process is obtained from the system 600 because of the third classification zone and associated side stream.

The principle of operation of the crystallizer systems 500 and 606 illustrated in FIGS. and 6 of the drawings can be best illustrated by the reference to the concentration-temperature curves set forth in FIGS. 7 and 8 of the drawings. As is usual in such curves, the concentration shown therein is the concentration of dissolved salt in the solution and does not include any solids or crystals carried by, but not dissolved in, the solution. The curve shown in FIG. 7 is typical of prior art systems of crystallizing common salts, for example, magnesium sulfate, the line AB therein representing the solubility curve for the salt separating the undersaturation zone below the curve from the metastable zone above the curve. The dashed line CD is the so-called supersolubility curve or the hypothetical margin line dividing the metastable zone beneath the curve CD from the labile zone above the curve CD. In the undersaturation zone, there is no supersaturation in the solution available for crystal growth and in fact, there may be a net loss of crystals through dissolution thereof. In the metastable zone, there is supersaturation present in the solution available for crystal growth but not sufiicient to cause spontaneous formation of nuclei. In the labile zone, there is sufficient supersaturation present to cause spontaneous formation of nuclei with substantially no external cause therefor..

Point 701 on the curve AB represents the concentration of the re-cycling mother liquor in prior systems at the final crystallization temperature T this being an equilibrium point fixed by the operating conditions such as the pressure within the crystallizer. Point 762 represents the concentration of the initial feed solution at the feeding temperature T and the point 703 represents the resultant concentration achieved by mixing the recycling mother liquor and the initial feed and the temperature T resulting from the mixing of the two streams of difierent temperatures, the point 763 being positioned on a straight line connecting the points 701 and 702 and the position thereof along the line being determined by the relative weight of the individual component streams. The point 703 will move closer to the point 7&1 if more re-cycling mother liquor is mixed with a given weight of initial feed solution and conversely the point 793 will move closer to the point 702 if less re-cycling mother liquor is mixed with a given weight of initial feed solution. In a typical prior art crystallization process, the solution represented by the point 703 is passed through a heater and is heated to a temperature T; represented by the point 704. If there are no appreciable solids to dissolve in the mother liquor, there will be no significant change in concentration and the line interconnecting the points 703 and 704 will be substantially horizontal; on the other hand, if there is dissolution O1 solids, then the concentration will slightly increase in going from point 703 to point 704. The heated solution represented by the point 704 is then fed into the crystallizer and it will flash to an equilibrium temperature T determined by the pressure within the crystallizer, the flashing of the solution removing the solvent as vapor and causing supersaturation of the remaining solvent. There is supersaturation present when changing the temperature from T to T along the line interconnecting the points 704 and 705 as soon as the temperature crosses the curve AB and the supersaturation will increase until the temperature T is reached at which time no further fiashing will take place. As soon as there is any supersaturation in the solution, there will be crystal growth on any crystal surfaces existing in the solution. Care must be taken in the correlating of the temperature T with the pressure within the crystallizer to insure that the point 705 lies in the metastable zone and not the labile zone since there will be an undersired spontaneous formation of nuclei as soon as the labile zone is reached.

In the desupersaturation or crystal growth process, the temperature of the solution will increase as the solution flows downwardly through the crystallizer if there is no way to dissipate the heat of crystallization evolved. Accordingly, crystal growth and desupersaturation is accompanied by a slight rise in temperature from T to T and a corresponding reduction in concentration from point 705 to 701. The temperature dilference between T and T must be minimized since the yield per cycle is decreased as the temperature difierence increases due to the fact that the curve AB goes upwardly indicating that less supersaturation is utilized in crystal growth as the temperature T increases.

The degree of supersaturation is the difference between the concentration at the point 705 and the concentration at the same temperature T on the curve AB, namely, that represented by the point 705'. Preferably, the degree of supersaturation is minimized in order to achieve a crystal product of the desired size, and in the conventional process illustrated in FIG. 7, this is accomplished by resorting to re-cycling a large volume of mother liquor. This procedure has serious disadvantages in that 11 the mother liquor Withdrawn from the crystallizer will contain only small amounts of solids and consequently more facilities will be required to remove the crystal applied to the crystallization of the same solution as in FIG. 7, the temperatures T T and T and the corresponding concentrations in FIG. 8 being the same as the like temperatures and concentrations in FIG. 7. More specifically, the point 8'31 represents the concentration and temperature of mother liquor in the pipe 553 of FIG. 5, the point 852 represents the temperature and concentration of the initial feed solution in the pipe 501 and the point 863 represents the first mixing of the mother liquor and the initial feed solution. This first mix: is further joined by the first stage circulation solution which has a concentration and temperature represented by the point 884, this solution being contained in the pipe 517 in FIG. 5. The solution at the point 894 is still supersaturated and has a concentration lying on the line interconnecting the points 559 and Sill. The mining of the solutions in the pipes 591, 537 and 553 provides a solution in the pipe 592 having the temperature and concentration represented by the point 855. That Solution is then fed into the heater 593 where it is heated to the temperature T The nuclei contained in the first side stream solution appearing in the pipe 517 are dissolved as are a portion of any crystals remain- 7 ing in the mother liquor in the pipe 553 and, accordingly, there is a slight increase in concentration in going from the point 895 to the point 8%. The temperature T in FIG. 8 is identical to the temperature T 4 in FIG. 7 and is sufficiently high to dissolve the nuclei in the solution provided that sufiicient time is permitted to achieve dissolution.- Those crystals having the greater proportional surface area, i.e., the nuclei, will be preferentially dissolved since the rate of dissolution is proportional to the surface areaand in fact the nuclei will be substantially completely dissolved before the larger crystals start to dissolve. The larger crystals thus saved provide seeds for crystal growth in the crystallizer 1% and increase the economy of the ciystallizing process.

The heated solution represented by the point 8% is fed to the pipe 504- where it is mixed with the two side streams in the pipes 527 and 537 which comprise the recirculation magma from the second stage and the bottom of the crystallizer The combined recirculation streams in the pipes 527 and 537 have a concentration and temperature represented by the point 897, these solutions carrying a small amount of supersaturation at a temperature lower than the temperature T The resultant stream entering the connection 10-3 has a temperature and concentration at the point see, the position of the point 898 on the line connecting the points 806 and 807 being determined by the ratio between the volume of solution leaving the heater 533 and the volume of solution in the pipes 527 and 537. Preferably this ratio is arranged so that the recirculation streams in the pipes 527 and 537 cool the solution at the point 893 into the undersaturation zone but at a temperature T only slightly less than that required to enter the metastable zone.

Entry of the stream through the connection 163 into the areaof reduced pressure in the crystallizer 100 causes a portion of the solution to fiash at a temperature T which is identical to the temperature T in FIG. 7. The slope of the line interconnecting the points 794 and 705 is the same as that interconnecting the points 808 and 899 provided the solvents are the same. As a result of the flashing, the remaining solution will be cooled to the temperature T and will have a concentration such that it is supersaturated, the degree of supersaturation being indicated by the difference of concentrations between the points 899 and 559' at T The supersaturated solution will desupersaturate along theline 89% 801 as the solution fall through the crystallizer 166. The completely desupersaturated solution in dynamic equilibrium with the solid crystals, is. the mother liquor represented by the point 891, is present at the bottom connection 134 and the mother liquor is removed and recirculated through the pipe 553 as describedabove.

From the above description of the cycle illustrated in FIG. 8, it is clear that a minimum degree of supersaturation is utilized in the present system, namely that rep resented by the difference in concentration between the points 369 and 8M and that this degree of supersaturation is substantially less than that of the conventional system illustrated in FIG. 7 when utilizing like operating conditions, i.e., the same temperature and concentration of the mother liquor, the same temperature and concentration of the initial feed solution, the same temperature of the solution leaving the heater, and the same flashing temperature. The initial crystallization point 899 in the process of FIG. 8 is also nfurther removed from the labile zone than is the corresponding point 785 in the process of FIG. 7 and consequently there Will be a minimum of spontaneous formation of new nuclei in each cycle.

There is shown in FIG. 9 of the drawings a slightly modified system 900 embodying the present invention and illustrating the crystallizer 100 utilized to crystallize ammonium sulfate from a solution obtained by reacting concentrated sulfuric acid and ammonia gas. In the system 900, concentrated sulfuric acid is fed in a pipe 901 (from a source not shown) through a control valve 901a to a mixing pipe 902. The pipe 902 connects with a reactor 904 which has connections to receive ammonia gas through a supply pipe 903 (connected to a source not shown) through a control valve 903a. The concentrated sulfuric acid and the ammonia gas chemically react in the apparatus 904 to form an ammonium sulfate solution which is fed through a pipe 905 to the inlet connection 103 of the crystallizer 190. The first side stream taken from thecrystallizer 100 through the outlet connection 114 is conveyed through a pipe 915 under the action of a pump 916 through a pipe 917 having a control valve 917a therein and communicating with the pipe 902 ahead of the sulfuric inlet pipe 901 so that the concentrated sulfuric acid can be injected into a stream for dilution therein. The second side stream taken from the crystallizer 100 through the outlet connection 124 is conveyed through a pipe 925 under the actionof a pump 926 to a line 927 connecting with the pipe 902 after injection of the sulfuric acid therein, the pipe 927 having a control valve 927a therein. The third side stream derived from the crystallizer 100 through the outlet connection 133 is conveyed through a pipe 935 to a pump 936 and then to a pipe 937 connecting with the pipe 905, the pipe 937 having a valve 937a therein. Mother liquor containing crystal product is withdrawn through the bottom cormection 134 and conveyed by a pipe 945 to a pump 946 and by a pipe 947 having a control valve 937a therein to a crystal separation apparatus 907 which may be a centrifuge or filter as described above. The Wet product from the apparatus 907 is removed through a conduit 908 and the mother liquor is conveyed through a pipe 910 to a junction with a makeup water pipe 906 having a control valve 906a therein. The resulting mixed stream is fed through a pipe 909 having a control valve 909:: and a check valve therein to the pipe 902.

In the system 900' the crystallizer operates as has been explained above with respect to FIG. 5 but the various side streams therefrom are utilized in a somewhat different manner to accommodate the particular needs of the system in manufacturing ammonium sulfate from the conthe pipe 985'. centration of ammonitun sulfate in the solution falls and centrated sulfuric acid and ammonia gas. The three streams in the pipes 936, 910 and 917 are mixed together to form a relatively large stream to which is added a relatively small stream of concentrated sulfuric acid whereby to dilute the sulfuric acid, the dilution of the sulfuric acid being accompanied by some evolution of heat. The stream in the pipe 917 may contain substantial nuclei and these are at least parially dissolved by the heat evolved by the dissolution of the sulfu-ic acid in the stream within the pipe 962. The heated solution is then partially cooled by the second side stream from the pipe 927 before the ammonia gas is added thereto. Dissolution of the ammonia gas in the solution from the pipe 992 serves substantially to heat the solution whereby further to destroy nuclei therein. The third side stream in the pipe 957 is admitted to the pipe $85 to cool the solution coming from the reactor 9114 and the resultant stream constitutes the feed for the crystallizer 1%.

There is shown in FIG. 10 of the drawings a concentration-temperature diagram of the crystallization process carried out by the system da'l of FIG. 9, the curve EF being the solubility curve of ammonium sulfate in water, the temperatue being plotted in degrees Fahrenheit along the horizontal axis and the concentration of ammonium sulfate dissolved in solution, in weight percent, being plotted along the vertical The point 1961 represents the equilibrium solution of the mother liquor in the pipe 910 and corresponds to the point 801 in FIG. 8. The initial feed solution in FIG. 10 (corresponding to the point SC-*2 in FIG. 8) is actually a mixture of make-up water and sulfuric acid which has an ammonium sulfate concentration of at 122 P. which is off of the graph in HQ. 10 but is symbolically represented thereon at 16112. The of the mother liquor in the pipe 919 with the sulfuric acid and the make-up Water produces a diluted mother liquor having a concentration and temperature represented by the point 1 383 (corresponding to the point 8113 in FIG. 8). To this is added the first side stream containing nuclei from rthe pipe 917 having a concentration and temperature represented by the point 10% (corresponding to the point 894 in FIG. 8) and the resulting mixture has a temperature and concentration represented by the point 18%5, this being the solution in the pipe 992 before the addition of the stream from the pipe 927 (corresponding to point 805 in FIG. 8). To this is then added the second side stream through the pipe 927, the side stream having a temperature and concentration represented by the point 169%. The resulting mixture entering the reaction chamber 984 has the temperature and con centration represented by the point 1907.

The reaction of the ammonia gas with the sulfuric acid solution evolves substantial heat and raises the temperature of the solution to that represented by the point 1908 (corresponding to the point 806 in FIG. 8) and slightly increases the concentration of ammonium sulfate in solution by dissolving the nuclei therein. To the heated solution is then added the third side stream from the pipe 937 which has the composition and temperature represented by the point 1069 (corresponding to the point S97 of FIG. 8) and the resultant mixture has a temperature and concentration represented by the point 16 10 (corresponding to point 8423 in FIG. 8), this solution being fed through the pipe 995 to the inlet connection 1&3.

Flashing of the solution as it enters the crystallizer 1% removes solvent from the solution whereby to raise the concentration of ammonium sulfate in the remaining solution and to reduce the temperature to that represented by the point 1811 (corresponding to point 899 in FIG. 8). The remaining solution is now supersaturated since point 1%11 lies in the metastable zone and immediately desupersaturation begins accompanied by crystal growth on the seed crystals fed into the crystallizer 196 through As desupersaturation proceeds, the conthe temperature slightly increases along the line 1e11- 11361. The first side stream is removed at the operating point 1064- through the connection 114, the second side stream is removed at the operating point 1006 through the outlet connection 124, the third side stream is removed at the operating point 18115 through the outlet connection 133 and the mother liquor with the crystal product therein is removed from the bottom connection 134 at the operating point 16191. This completes a cycle of operation of the system 960.

The above desired operation of the system 9th} is achieved by utilizing apparatus having the following dimensions and operating under the conditions here set forth. It is to be understood that this specific example is given only by way of illustration'and is not intended in any way to limit the present disclosure.

The system can be designed to operate utilizing as raw materials 98% by Weight of sulfuric acid at the rate of 12,800 pounds per hour and anhydrous ammonia at 50 F. and 75 pounds per square inch gage pressure in the amount of 4,378- pounds per hour. The mother liquor drawn from the bottom outlet connection 134 has a temperature of 146 F. and an ammonium sulfate concentration of 47% by weight (corresponding to point 19131 in FIG. 10) and the crystallizer is operated at a pressure of 25 inches of mercury vacuum. When operated in the above manner, the system 900 will produce 203 tons per day of ammonium sulfate (assuming a 1.5% over-all loss) having a crystal size such that 60% will be maintained on a 16 mesh screen and 90% retained on a 28 mesh screen.

The crystallizer lllil has the following dimensions when utilized in the system 91251: the diameter of the vapor head 162 is 9 feet and the height thereof is 9 feet; the diameter of the body 191 is 16 feet and the height thereof is 24 feet; the slope of the sides of the funnels and is 60; the diameter of the neck 1-11 is 3 feet, 6 inches and the length thereof is 3 feet, 6 inches; the diameter of the neck 121 is 2 feet, 8 inches and the length thereof is 5 feet, 4 inches; the baffle 112 is inclined at an angle of 30 with respect to the horizontal and has an opening therein of 7 feet, 3 inches and the radius of the passage 115 is 7 /2 inches; the angularity of the baffle 112 is also 30 and the diameter of the opening therein is 10 feet, 7 inches and the radius of the annular passage is 3 /2 inches; the manifold or collector ring 113 has ten 6 inch diameter openings therein communicating with the interior of the crystallizer and the collector ring 123 has fourteen 6 inch diameter openings therein; the entry baffle 105 has a dimeter of 7 feet, 10 inches and is placed 9 inches away from the inlet connection 1% and 5 inches away from the nearest wall of the vapor head 102, the lower edge of the baffle 105 being disposed 3 feet above the juncture of the vapor head 1132 and the wall section 116.

The crystallizer having the dimensions described above has a suspension volume for solution of 5,950 cubic feet. The circulation rate from the first stage through the outlet connection 114 is adjusted to be 2900 gallons per minute, the circulation rate from the second stage through the outlet connection 124 is 3800 gallons per minute and the circulation rate from the bottom through the outlet connections 133 and 134 is 4,855 gallons per minute. The flow velocity at the inlet connection 1%3 will be 7.4 feet per second. The pump 9 16 and the control valve 917a are adjusted so that the flow velocity through the outlet connection 114 for the first stage is 7.8 feet per second which provides a superficial flow velocity for nuclei in the first stage classification zone of 0.034 foot per second. The pump 926 and the control valve 927a are adjusted to provide a flow velocity of 8.5 feet per second through the second stage outlet connection 124 which results in a superficial flow velocity of fines in the second stage classification size of 0.043 foot per second. The bottom outlet connection 134 has a flow velocity therethrough of 8.1 -feet per second. The above flow rates result in a flow velocity of two feet per second through the first stage neck 111 and a flow velocity of two feet per second through the second stage neck 121.

The cross sectional area of the classification zone in the first stage is 191 square feet and the distance from the bottom of the funnel 111 to the passage 115 is 6 feet. The cross sectional area of the classification zone in the second stage is 194 square feet and the distance from the bottom of the neck 121 to the passage 12-5 is 12 feet.

Recapitulatingythe classifying crystallizer of the present invention includes at' least two, and if desired, three or more stages contained within a shell provided with inlet and outlet connections so that a main-stream of crystallizable solution can be fed through the various stages in succession. The crystalliza-ble solution may contain supersaturation or the crystallizer can be provided with a connection to an area of reduced pressure to flash a portion of the solution and cause supersaturation of the remainder thereof whereby the supersaturated solution as it flows through the various stages has formed therein nuclei and fines and undersized crystals and crystal product in mother liquid by desupersaturation of the solution and crystal growth. An improved entry bafiie has been provided to minimize disturbance and splashing of the supersaturated solution thereby substantially to reduce the number of nuclei formed. Each of the stages in the crystallizer includes a classification zone through which a side stream is withdrawn, each side stream containing a predetermined size range of solids, i.e., the first side stream from the first stage (the stage nearest the. inlet) carrying substantially only nuclei from the first classification zone, and the second and successive side streams from the second and successive stages carry- 7 ing crystals smaller than crystal product and of increasing size in successive stages. To accomplish this, each stage comprises a funnel extending downwardly and cooperating with an outlet conduit spaced upwardly from the lower end of the funnel to provide therebetween a classification zone and preferably a baffie or a weir ring is provided between the lower end of the funnel and the outlet conduit to provide a passage of controlled area. The rate of flow in the classification zones increase successively from the inlet to the outlet of the crystallizer, and the length and cross sectional area of the classification zones increase from the inlet to the outlet.

The crystallizer as described above is further incorporated in systems of the present invention which include means such as a heater to remove the nuclei from the first side stream; the first side stream, the mother liquor obtained from crystal product recovery and the initial feed solution preferably are all heated preferentially to dissolve the nuclei and the colder side streams from the second and subsequent stages are mixed with the heated stream to provide an inlet stream for the crystallizer having the desired temperature and salt concentration. Suitable controls are provided to obtain the desired characteristics of operation of the system and particularly the desired rate of flow of the inlet streams, the various outlet or side streams, and the product stream.

In view of the foregoing, it is apparent that there has been provided an improved classifying crystallizer and an improved crystallizing system incorporating the improved classifying crystallizer therein which satisfies all of the objects and advantages set forth above.

While there has been described what is at present considered to be certain preferred embodiments of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A classifying crystallizer system for separating crystals from solutions, comprising a crystallizer shell, an inlet pipe communicating with said shell for supplying solution from which crystals are to be crystallized, an outlet pipe communicating with said shell and supplied with mother liquor containing crystal product, whereby passage of the solution from said inlet pipe in a main stream through said shell to said outlet pipe serves to form nuclei and fines and undersize crystals and crystal product in mother liquor by desupersaturation of the solution and crystal growth, first conduit means communicating with-said shell for separating a first side stream containing substantially onlynuclei from the main stream adjacent to said inlet pipe, second conduit means communicating with said shell between said first conduit means and said outlet pipe for separating a second side stream containing crystals smaller than the crystal prodnot from the main stream between said. first conduit means and said outlet pipe, means connected to said first conduit to remove the nuclei from the first side stream, and means connecting said first and second conduit means to said inlet pipe to return the first and second side streams to the main stream through said inlet pipe.

2. The classifying crystallizer system set forth in claim 1, wherein the means connected to the first conduit means comprises means to heat the first side stream to dissolve the nuclei therein.

3. A classifying crystallizer system for separating crystals from solutions, comprising a crystallizer shell, an inlet pipe communicating Wlllhihfi upper portion of said shell for supplying a solution from which crystals are to be crystallized, a vapor outlet communicating with the upper portion of said shell above said inlet pipe for connecting to an area having apressure lower than that in said shell, a plate mounted in said shell wd extending thereacross and disposed between said inlet pipe and said vapor outlet and having an opening therein, a substantially cylindrical bafile mounted on said plate and surrounding the opening therein and extending downwardly past said inlet pipe, said bafile being disposed nearer the wall of said shell opposite said inlet pipe to provide more flow space on the inlet side of said bafile, whereby the solution entering from said inlet pipe irnginges upon said bafiie to flow therearound and downwardly therealong and to fall therefrom in a continuous liquid curtain, an outlet pipe communicating with said shell and supplied with mother liquor containing crystal product, whereby passage of the solution from said inletpipe into said shell and around and down said bafile is accompanied by evaporation of a portion of the solution to form vapor which passes upwardly through the opening in said plate and through said vapor outlet to provide supersaturation in the remaining portion of the solution which flows through said shell in a main stream in which are formed nuclei and fines and undersize crystals and crystal product in mother liquor by desupersaturation and crystal growth, conduit means communicating with said shell below said bafile for separating a side stream containing substantially only nuclei from the main stream, means connected to said conduit to remove the nuclei from the side stream, and means connecting said conduit to said inlet pipe to return the side stream to said shell through said inlet pipe.

4. The classifying crystallizer system set forth in claim 3, wherein the upper portion of the shell is substantially circular in cross section, the plate is inclined downwardly away from the inlet pipe, and the baflie is substantially circular in cross section and is arranged eccentrically with respect to the upper portion of said shell and farthest away from said inlet pipe. 7 I

5. A classifying crystallizer system for separating crystals from solutions, comprising means for heating a solution having an inlet and an outlet, a supply pipe interconnecting said inlet and a source of initial feed solution, a crystallizer shell, an inlet pipe interconnecting said outlet and the upper portion of said shell, a vapor outlet communicating with the upper portion of said shell above said inlet pipe for connecting to an area having a pressure lower than that in said shell, an outlet pipe communicating with said shell and supplied with mother liquor containing crystal product, whereby passage of the solution from said inlet pipe into said shell is accompanied by evaporation of a portion of the solution to form vapor which passes upwardly through said vapor outlet to provide supersaturation in the remaining portion of the solution which flows through said shell in a main stream in which are formed nuclei and fines and undersize crystals and crystal product in mother liquor by desupersaturation and crystal growth, first conduit means communicating with said shell for separating a first side stream containing substantially only nuclei from the main stream adjacent to said inlet pipe, second conduit means communicating with said shell between said first conduit means and said outlet pipe for separating a second side stream containing crystals smaller than the crystal product from the main stream between said first conduit means and said outlet pipe, means connected to said outlet pipe for removing crystal product from the mother liquor therein, and means connected to said first and second conduit means and said crystal product removing means to return the first and second side streams and the mother liquor to the main stream through said inlet pipe, at least said first conduit means being connected to said supply pipe whereby the first side stream passes through said heating means to dissolve the nuclei therein.

6. The classifying crystallizer system set forth in claim 5, wherein the second conduit means is connected to the inlet pipe, and the crystal product removing means is connected tothe supply pipe.

7. The classifying crystallizer system set forth in claim 5, wherein a third conduit means is connected to the outlet pipe to return a portion of the mother liquor with crystal product therein to the main stream through the inlet pipe.

8. The classifying crystallizer system set forth in claim 7, wherein the second and third conduit means are connected to the inlet pipe, and the crystal product removing means is connected to the supply pipe.

-9. The classifying crystallizer system set forth in claim 7, wherein the third conduit means is connected to the inlet pipe, and the second conduit means and the crystal product removing means are connected to the supply pipe.

10. A classifying crystallizer system for separating crystals from solutions, comprising a source of initial feed solution, a heater having an inlet and an outlet, a supply pipe interconnecting said source and said heater inlet, a

crystallizer shell, means dividing said shell into upper and intermediate and lower chambers, an inlet pipe interconnecting said heater outlet and said upper chamber, a vapor outlet communicating with the upper portion of said shell above said inlet pipe for connecting to an area having a pressure lower than that in said shell, an outlet pipe communicating with the lower portion of said lower chamber, whereby passage of solution from said inlet pipe into said shell is accompanied by evaporation of a portion of the solution to form vapor which passes upwardly through said vapor outlet to provide supersaturation in the remaining portion of the solution which fiows through said chambers in a main stream in which are formed nuclei and fines and undersized crystals and crystal product in mother liquor by desupersaturation and crystal growth, means in said intermediate chamber providing a first classification zone and means in said lower chamber providing a second classification zone, first conduit means communicating with said intermediate chamber for withdrawing a first side stream containing substantially only nuclei from the main stream through said first classification zone, second conduit means communicating with said lower chamber for withdrawing a second side stream containing crystals smaller than crystal product from the main stream through said second classification zone, means connecting said outlet pipe to said heater inlet to return the mother liquor thereto, means connecting said first conduit means to said heater inlet to return the first side stream thereto, and means connecting said second conduit means to said inlet pipe to return the second side stream thereto.

11. The classifying crystallizer system set forth in claim 10, wherein adjustable flow control means are provided in said supply pipe and in said first and second conduit means and in said outlet pipe, whereby the rate of flow of the first and second side streams through the first and second classification zones can be adjusted to remove thereby a predetermined size range of nuclei and fines and crystals.

12. A classifying crystallizer system for separating crystals from solutions, comprising an upstanding shell, a first downwardly extending funnel disposed in said shell and a second downwardly extending funnel disposed in said shell below and spaced from said first funnel, said funnels dividing said shell into upper and intermediate and lower chambers, said first funnel having a first opening in the lower portion thereof communicating with said intermediate chamber and said second funnel having a second opening in the lower portion thereof communicating with said lower chamber, means for heating a solution, a supply pipe interconnecting said heating means and a source of initial feed solution, an inlet pipe interconnecting said heating means and said upper chamber, a vapor outlet communicating with the upper portion of said shell above said inlet pipe for connecting to an area having a pressure lower than that in said shell, a first outlet in the upper portion of said intermediate chamber and spaced vertically above said first opening to provide therebetween a first classification zone, a second outlet in the upper portion of said lower chamber and spaced vertically above said second opening to provide therebetween a second classification zone, a third outlet in the lower portion of said lower chamber and spaced vertically below said second opening, whereby passage of the heated solution from said inlet pipe into said shell is accompanied by evaporation of a portion ofthe solution to form vaporwhich passes upwardly through said vapor outlet to provide supersaturation in the remaining portion of the solution which flows through said chambers in a main stream and to'said outlets to form nuclei and fines and undersized crystals and crystal product in mother liquor by desupersaturation and crystal growth, first conduit means connected to said first outlet for withdrawing a first side stream containing substantially only nuclei fnom the main stream through said first classification zone, second conduit means connected to said second outlet for withdrawing a second side stream containing crystals smaller than the crystal product from the main stream through said second classification zone, means connected to said third outlet for removing crystal product from the mother liquor, and means connected to said first and second conduit means and said crystal product removing means to return the first and second side streams and the mother liquor to the main stream through said inlet pipe, at least said first conduit means being connected to said supply pipe whereby the first side stream passes through said heating means to dissolve the nuclei therein.

13. The classifying crystallizer system set forth in claim 12, wherein the flow velocity of the first side stream through the first classification zone is less than that of the second side stream through the second classification zone, and the length of the flow path in the first classification zone is less than that in the second classification zone.

14. The classifying crystallizer system set forth in claim 13, wherein the first opening in the first funnel has a lar er area than the second opening in the second funnel, and the cross sectional area of the first classification zone is less than that of the second classification zone. 1

15. The classifying crystallizer system set forth in claim 12, wherein a first baffle is mounted in the shell between the first opening in the first funnel and the first outlet to provide a first predetermined flow passage, and a second bafile is mounted in the shell between the second opening in the second funnel and the second outlet to provide a second predetermined flow passage.

16. A classifying crystallizer system for separating a first downwardly extending funnel disposed in said shell and a second downwardly extending funnel disposed in said shell below and spaced from said first funnel and a third downwardly extending funnel disposed in said shell below and spaced from said second funnel, said funnels dividing said shell into upper and upper intermediate and lower intermediate and lower chambers, said first funnel havinga first opening in the lower portion thereof communicating with said upper intermediate chamber and said second funnel having a second opening in the lower portion thereof communicating with said lower intermediate chamber and said third funnel having a third opening in the lower portion thereof communicating with said lower chamber, means for heating a solution, 'a supply pipe interconnecting said heating means and a source of initial feed solution, an inlet pipe interconnecting said heating means and said upper chamber, a vapor outlet communicating with the upper portion of said shell above said inlet pipe for connecting to an area having a pressure lower than that in said shell, a first outlet in the upper portion of said upper intermediate chamber and spaced vertically above said first opening to provide therebetween a first classification zone, a second outlet in the upper portion of said lower intermediate chamber and spaced vertically above said second opening to provide therebetween a second classification zone, a third outlet in the upper portion of said lower chamber and spaced vertically above said third opening to provide therebetween a third classification zone, a fourth outlet in the lower portion of said lower chamber and spaced vertically below said third opening, whereby passagetoftthe heated solution from said inlet pipe into said shell is accompanied by evaporation of a portion of the solution to form vapor which passes upwardly through said vapor outlet to provide supersaturation in the remaining portion of the solution which flows through said chambers in a main stream and to said outlets to form nuclei and fines and undersized crystals and crystal product in mother liquor by desupersaturation and crystal growth, first conduit means connected to said first outlet for withdrawing a first side stream containing substantially only nuclei from the main stream through said first classification zone, second conduit means connected to said second outlet for withdrawing a second side stream containing fines and crystals smaller than the crystal product from the main stream through said second classification zone, third conduit means connected to said third outlet for withdrawing a third side stream containing crystals larger than those in said second side stream and smaller than the crystal product from the main stream through said third classification zone, means connected to said fourth outlet for removing crystal product from the mother liquor, and means connected to said first and second and third conduit means and said crystal product removing means to return the first and second and third side streams and the mother liquor to the main stream crystals from solutions, comprising an upstanding shell,

through said inlet pipe, at least said first conduit means a being connected to said supply pipe whereby the first side stream passes through said heating means to dissolve the nuclei therein 17. A classifying crystallizer for separating crystals from solutions, comprising an upstanding shell, a first downwardly extending funnel disposed in said shell and a second downwardly extending funnel disposed in said shell below and spaced from said first funnel, said funnels dividing said shell into upper and intermediate and lower chambers, said first funnel having a first opening in the lower portion thereof communicating with said intermediate chamber and said second funnel having a second opening in the lower portion thereof communicating with said lower chamber, an inlet in said upper chamber for solution from which crystals are to be crystallized, a first outlet in the upper portion of said intermediate chamber and spaced vertically above said first opening to provide therebetween a first classification zone, a second outlet in the upper portion of said lower chamber and spaced vertically above said second opening to provide therebetween a second classification zone, and a third outlet in the lower portion of said lower chamber and spaced vertically below said second opening, whereby passage of the solution from said inlet through said chambers and said openings to said outlets serves to form nuclei and fines and undersize crystals and crystal product in mother liquor by desupersaturation of the solution and crystal growth, said openings and said outlets being arranged to remove nuclei in said first classification zone through said first outlet and to remove crystals smaller than the crystal product in said second classification zone through said second outlet and to supply mother liquor containing crystal product through said third outlet.

18. The classifying crystallizer set forth in claim 17, wherein the distance from said first opening to said first outlet is less than the distance from said second opening to said second outlet.

19. The classifying crystallizer set forth in claim' 18, wherein the diameter of said first opening is greater than the diameter of said second opening, and the cross sectional area of the first classification zone is less than the cross sectional area of the second classification zone.

20. A classifying crystallizer for separating crystals from solutions, comprising an upstanding shell, a first downwardly extending funnel disposed in said shell and a second downwardly extending funnel disposed in said shell below and spaced from said first funnel, said funnels dividing said shell into upper and intermediate and lower chambers, said first funnel having a first opening in the lower portion thereof communicating with said inter-mediate chamber and said second funnel having a second opening in the lower portion thereof communicating with said lower chamber, an inlet in said upper chamber for solution from which crystals are to be crystallized, a first outlet in the upper portion of said intermediate chamber and spaced vertically above said first opening, a first bafie mounted in said shell between said first opening and said first outlet to provide a predetermined fiow passage and to define a first classification zone between said first opening and said first baffle, a second outlet in the upper portion of said lower chamber and spaced vertically above said second opening, a second bafiie mounted in said shell between said second opening and said second outlet to provide a predetermined fiow passage and to provide a second classification zone between said second opening and said second balfle, a third outlet in the lower portion of said lower chamber and spaced vertically'below said second 21 said second outlet and to supply mother liquor containing crystal product through said third outlet.

21. The classifying crystallizer set forth in claim 20, wherein each of the baffles is mounted on the shell around the periphery thereof and is inclined upwardly toward the associated funnel and is spaced therefrom to provide therebetween an annular flow passage of predetermined flow area.

22. The classifying crystallizer set forth in claim 20, wherein each of the bafiles is mounted on the shell around the periphery thereof and comprises a weir ring surrounding the associated funnel and spaced therefrom to provide a passage therebetween of predetermined flow area.

23. A classifying crystallizer for separating crystals from solutions, comprising an upstanding shell, 21 first downwardly extending funnel disposed in said shell and a second downward-1y extending funnel disposed in said shell below and spaced from said first funnel, said funnels dividing said shell into upper and intermediate and lower chambers, an inlet in said upper chamber for solution from which crystals are to be crystallized, a vapor conduit communicating with the upper portion of said chamber above said inlet for connecting to an area having a pressure lower than that in said shell, a plate mounted in said upper chamber and extending thereacross and disposed between said inlet and said vapor conduit and having an opening therein, a substantially cylindrical baffle mounted on said plate and surrounding the opening therein and extending downwardly past said inlet, said bafile being disposed nearer the wall of said upper chamber opposite said inlet to provide more fiow space on the inlet side of said bafile, whereby the solution entering from said inlet impinges upon said baffle to flow therearound and downwardly therealong and to fall there-from in a continuous liquid curtain, a first outlet in the upper portion of said intermediate chamber and spaced vertically above said first opening to provide therebetween a first classification zone, a second outlet in the upper portion of said lower chamber and spaced vertically above said second opening to provide therebetween a second classification zone, and a third outlet in the lower portion of said lower chamber'and spaced vertically below said second opening, whereby passage of the solution from said inlet through said chambers and said openings to said out-lets serves to form nuclei and fines and undersize crystals and crystal product in mother liquor by desupersaturation of the solution and crystal growth, said openings and said outlets being arranged to remove nuclei in said first classification zone through said first outlet and to remove crystals smaller than the crystal product in said second classification zone through said second outlet and to supply mother liquor containing crystal product through said third outlet.

24. The classifying crystallizer set forth in claim 23, wherein the upper chamber of the shell is substantially circular in cross section, the plate is inclined downwardly away from the inlet, and the baffle is substantially circular in cross section and is arranged eccentn'cally with respect to said upper chamber and farthest away from said inlet 25. A classifying crystallizer for separating crystals from solutions, comprising an upstanding shell, a first downwardly extending funnel disposed in said shell and a second downwardly extending funnel disposed in said shell below and spaced from said first funnel and a third downwardly extending funnel disposed in said shell below and spaced from said second funnel, said funnels dividing said shell into upper and upper intermediate and lower intermediate and lower chambers, said first funnel having a first opening in the lower portion thereof communicating with said upper intermediate chamber and said second funnel having a second opening in the lower portion thereof communicating with said lower intermediate chamber and said third funnel having a third opening in the lower portion thereof communicating with said lower chamber, an inlet in said upper chamber for solution from which crystals are to be crystallized, =a first outlet in the upper portion of said upper intermediate chamber and spaced vertically above said first opening to provide therebetween a first classification zone, a second outlet in the upper portion of said lower intermediate chamber and spaced vertically above said second opening to pro vide therebetween a second classification zone, a third outiet in the upper portion of said lower chamber and spaced vertically above said third opening to provide therebetween a third classification zone, and a fourth outlet in the lower portion of said lower chamber and spaced vertically below said third opening, whereby passage of the solution from said [inlet through said chambers and said openings to said outlets serves to form nuclei and fines and undersize crystals and crystal product in mother liquor by desupersaturation of the solution and crystal growth, said openings and said outlets being ar ranged to remove nuclei in said first classification zone through said first outlet and to remove crystals smaller than the crystal product in said second classification zone through said second outlet and to remove crystals larger than the crystals removedin said second classification zone and smaller than the crystal product in said third classification zone through said third outlet and to supply mother liquor containing crystal product through said fourth outlet. 7

26. A classifying crystallizer as set forth in claim 25, wherein the vertical extents of the classification zones become successively greater from the first classification zone toward the third classification zone, the diameters of the funnel openings become successively smaller from the first classification zone to the third classification zone, and the cross sectional flow areas of the classification zones become successively greater from the first classification zone to the third classification zone.

References Cited in the file of this patent UNITED STATES PATENTS 1,873,251 Allen Aug. 23, 1932 2,631,926 Eckstrom Mar. 17, 1953 2,808,103 Cook Oct. 1, 1957 2,883,273 Saeman Apr. 21, 1959 

1. A CLASSIFYING CRYSTALLIZER SYSTEM FOR SEPARATING CRYSTALS FROM SOLUTIONS, COMPRISING A CRYSTALLIZER SHELL, AN INLET PIPE COMMUNICATING WITH SAID SHELL FOR SUPPLYING SOLUTION FROM WHICH CRYSTALS ARE TO BE CRYSTALLIZED, AN OUTLET PIPE COMMUNICATING WITH SAID SHELL AND SUPPLIED WITH MOTHER LIQUOR CONTAINING CRYSTAL PRODUCT, WHEREBY PASSAGE OF THE SOLUTION FROM SAID INLET PIPE IN A MAIN STREAM THROUGH SAID SHELL TO SAID OUTLET PIPE SERVES TO FORM NUCLEI AND FINES AND UNDERSIZE CRYSTALS AND CRYSTAL PRODUCT IN MOTHER LIQUOR BY DESUPERSATURATION OF THE SOLUTION AND CRYSTAL GROWTH, FIRST CONDUIT MEANS COMMUNICATING WITH SAID SHELL FOR SEPARATING A FIRST SIDE STREAM CONTAINING SUBSTANTIALLY ONLY NUCLEI FROM THE MAIN STREAM 